Steps to reproduce:

1. Create a new host without interfaces
2. Assign the Proxmox VE by HTTP template to it
3. Update macros for user and token
4. Note the failure to fetch data
5. Add an SNMP interface to the host using the IP or DNS
6. Note the HTTP items now populate correctly

Result:
502 error returned from proxmox.api.available
Expected:
OK 200 from proxmox.api.available

The documentation doesn't mention this requirement explicitly. Since the template won't work otherwise, it should be noted in the docs.